[serializer] clean up root list items.

src/snapshot/serializer.cc

 // Copyright 2016 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/snapshot/serializer.h"
 
 #include "src/macro-assembler.h"
 #include "src/snapshot/natives.h"
 
 namespace v8 {
@@ skipping 194 matching lines @@
 void Serializer::PutRoot(int root_index, HeapObject* object,
                          SerializerDeserializer::HowToCode how_to_code,
                          SerializerDeserializer::WhereToPoint where_to_point,
                          int skip) {
   if (FLAG_trace_serializer) {
     PrintF(" Encoding root %d:", root_index);
     object->ShortPrint();
     PrintF("\n");
   }
 
+  // Assert that the first 32 root array items are a conscious choice. They are
+  // chosen so that the most common ones can be encoded more efficiently.
+  STATIC_ASSERT(Heap::kEmptyDescriptorArrayRootIndex ==
+                kNumberOfRootArrayConstants - 1);
+
   if (how_to_code == kPlain && where_to_point == kStartOfObject &&
       root_index < kNumberOfRootArrayConstants &&
       !isolate()->heap()->InNewSpace(object)) {
     if (skip == 0) {
       sink_.Put(kRootArrayConstants + root_index, "RootConstant");
     } else {
       sink_.Put(kRootArrayConstantsWithSkip + root_index, "RootConstant");
       sink_.PutInt(skip, "SkipInPutRoot");
     }
   } else {
@@ skipping 565 matching lines @@
   if (to_skip != 0 && return_skip == kIgnoringReturn) {
     sink_->Put(kSkip, "Skip");
     sink_->PutInt(to_skip, "SkipDistance");
     to_skip = 0;
   }
   return to_skip;
 }
 
 }  // namespace internal
 }  // namespace v8